The invention relates to verification of the authenticity of an article such as a personal identification (ID) card, vendable product, original document or other item.
Many traditional authentication security systems rely on a process which is difficult for anybody other than the manufacturer to perform, where the difficulty may be imposed by expense of capital equipment, complexity of technical know-how or preferably both. Examples are the provision of a watermark in bank notes and a hologram on credit cards or passports. Unfortunately, criminals are becoming more sophisticated and can reproduce virtually anything that original manufacturers can do.
Because of this, there is a known approach to authentication security systems which relies on creating security tokens using some process governed by laws of nature which results in each token being unique, and more importantly having a unique characteristic that is measurable and can thus be used as a basis for subsequent verification. According to this approach tokens are manufactured and measured in a set way to obtain a unique characteristic. The characteristic can then be stored in a computer database, or otherwise retained. Tokens of this type can be embedded in the carrier article, e.g. a banknote, passport, ID card, important document. Subsequently, the carrier article can be measured again and the measured characteristic compared with the characteristics stored in the database to establish if there is a match.
Within this general approach it has been proposed to use different physical effects. One physical effect that has been considered in a number of prior art documents [1-4] is to use laser speckle from intrinsic properties of an article, typically in the form of a special token, to provide a unique characteristic. According to these techniques a large area, such as the whole of a special token, is illuminated with a collimated laser beam and a significant solid angle portion of the resultant speckle pattern is imaged with a CCD, thereby obtaining a speckle pattern image of the illuminated area made up of a large array of data points.
More recently a further laser speckle based technique has been developed [5] in which the unique characteristic is obtained by scanning a focused laser beam over the article and collecting many data points, typically 500 or more, from light scattered from many different parts of the article to collect a large number of independent data points. By collecting a large number of independent signal contributions specific to many different parts of the article, a digital signature can be computed that is unique to the area of the article that has been scanned. This technique is capable of providing a unique signature from the surfaces of a wide variety of articles, including untreated paper, cardboard and plastic.
An important application of this technique is security verification from a database of stored signatures, referred to as the “master database” in the following. For example, in a perfumery factory, each perfume bottle box can be scanned by a reader to obtain a signature, and these signatures are entered into a master database. The master database includes a signature from every article, i.e. box of perfume, produced. Later, for field verification, a reader can be used to scan any box of perfume to obtain a signature, and this signature is compared with the master database to establish whether there is a matching signature held in the master database. If there is no match, the article is considered to be counterfeit. If there is a match, then the article is considered to be genuine.
In many applications, for example those relating to national security, civil documentation or high volume branded goods, the number of signatures stored in the master database may be very large. The number of entries may be perhaps millions, tens of millions or even hundreds of millions. For example, this would be the case if the scheme is used for passport or driving licence verification for a populous country.
For most if not all applications, it is necessary that the search of the master database can be carried out in a reasonable time. What is reasonable will vary from application to application, but for many applications a maximum reasonable time will only be a few seconds. However, for large master databases, there are two difficulties in achieving a high speed search for a signature match.
Firstly, the scan even from a genuine item will never match its stored database scan perfectly. The test of a match or non-match is one of degree of similarity between the originally scanned signature held in the master database and the re-scanned signature. We find that a typical good quality match has approximately 75% of the bits in agreement, compared to an average of 50% agreement for a fraudulent match. Consequently, standard relational database fast searching methods such as look-up tables cannot be used efficiently. It is therefore necessary to try every entry in the database against the target signature.
Secondly, there may be an unknown bit-shift between the successful database entry and the rescanned signature. This is because the scanned object may not be in precisely the same position for the second scan as it was for the first scan. Any offsets in a direction parallel to the laser scan direction will result in a shifting of the bit pattern. It is therefore not only necessary to try every entry in the database against the target signature, but this must be done assuming a number of different lengths of bit-shifts for each database entry, which may be up to 30 or more, making the total search time potentially very long. The number of bit shifts is a function of the positioning accuracy of the readers and the per bit scan length.